Orthogonal Frequency-Division Multiplexing (OFDM) is a popular modulation technique which has many well documented advantages such as high spectral efficiency, immunity to frequency selective fading and high data rate transmission, see Cimini, L., Jr., “Analysis and Simulation of a Digital Mobile Channel Using Orthogonal Frequency Division Multiplexing,” IEEE Transactions on Communications, vol. 33, no. 7 pp. 665-675, July 1985.
Referring now to FIG. 1, there is shown schematically the main components of an ODFM transmitter. Input data 10 comprising voice related or data traffic is fed to an OFDM modulator 12 which distributes the data across a number of carriers 13 and for each carrier, sets an amplitude and phase value derived from the input data. A plurality of pilot signals are included among the data carriers 13, and some schemes also specify a number of unused carriers. The OFDM carrier data 13 can be conditioned 14 as described below before being converted to a time domain signal 15. This is in turn converted 16 to an analog signal before being upconverted 18 and then amplified 20 before being transmitted 22.
In OFDM systems, transmitted signal includes a sum of orthogonal sub-carriers that are modulated by complex data symbols. For an OFDM system with N sub-carriers, the transmitted baseband signal x(t) can be written as:
                                          x            ⁡                          (              t              )                                =                                    1                              N                                      ⁢                                          ∑                                  k                  =                  0                                                  N                  -                  1                                            ⁢                                                X                  k                                ⁢                                  ⅇ                                      j                    (                                          2                      ⁢                      π                      ⁢                                                                                          ⁢                      k                      ⁢                                                                                          ⁢                      Δ                      ⁢                                                                                          ⁢                      ft                                        )                                                                                      ,                  0          ≤          t          ≤          T                                    (        1        )            where Xk is the symbol carried by the kth sub-carrier, f is the frequency difference between sub-carriers, and T is the OFDM symbol duration. To ensure orthogonality among sub-carriers, the OFDM symbol duration should be T=1/Δf, the inverse of the frequency spacing of the sub-carriers. In the transmitter, the transmitted signal or sequence may be generated by the inverse fast Fourier transform (IFFT) of the N-point {Xk} sequencexn=IFFT(Xk)  (2)where the N-point sequence {Xk} is the sampled version of the transmitted signal x(t).
For example, an OFDM signal generated in IEEE 802.11a/HiperLAN2 Wireless LAN system comprises 64 sub-carriers, 48 of these are data carriers. Pilot sub-carriers are embedded in the OFDM signal among the data sub-carriers in order to enable channel estimation and frequency-offset tracking. There are 4 pilot sub-carriers for an IEEE 802.11a/HiperLAN2 Wireless LAN system. The remaining carriers are not used and have a value of zero.
A major disadvantage within OFDM systems is the rare occurrence of high peaks in the transmission signal. Such peaks in an OFDM signal can cause severe in-band and out-of-band distortion of a signal when entering the saturation region of the power amplifier 20. One way of quantifying the presence of the high peaks in a signal, is through measurement of the Peak-to-Average-Power Ratio (PAPR).
The PAPR of the transmitted signal can be expressed as:
                              P          ⁢                                          ⁢          A          ⁢                                          ⁢          P          ⁢                                          ⁢          R                =                  10          ⁢                                          ⁢                                    log              10                        (                                          max                ⁢                                                                                                x                      ⁡                                              (                        t                        )                                                                                                  2                                                            E                ⁡                                  [                                                                                                          x                        ⁡                                                  (                          t                          )                                                                                                            2                                    ]                                                      )                                              (        3        )            where E denotes the expectation. From equation (3), it will be seen that PAPR increases proportionally with the number of sub-carriers.
The simplest technique to combat this problem is to back off the operating point of the power amplifier 20 to accommodate extreme peaks. This can cause significant reduction in the transmission power impacting the efficiency of the amplifier—typical power efficiency of a class AB High Power Amplifier (HPA) is around 18% for an IEEE802.11a system, see Chen, K. C.; Morris, K. A.; Beach, M. A., “Increasing the power efficiency of an IEEE802.11a power amplifier,” IEEE 61st Vehicular Technology Conference, vol. 2, pp. 954-957, 30 May-1 Jun. 2005.
It is therefore preferable to reduce the Peak-To-Average-Power Ratio (PAPR) of the signal, without the need to back off the amplifier operating power.
For certain architectures including polar amplifier architecture it is also desirable to remove near-zero values of the amplitude signal. Polar architecture was designed by Kahn, L. R., “Single sideband transmission by envelope elimination and restoration,” Proc. IRE, vol. 40, pp 803-806, July 1952 for high amplifier efficiency. In polar systems the input signal is split into its amplitude and phase components. The phase modulation is generated digitally, up-converted and amplified with a nonlinear, highly efficient power amplifier. The amplitude modulation is added by modulating the supply voltage of the PA, thus yielding improved power efficiency.
If a near zero-amplitude value occurs in the OFDM signal, additional distortion will occur at the output of the polar amplifier or it may be forced to shut off, if the supply voltage becomes too low.
Various approaches to conditioning the signal prior to arriving at the amplifier have been proposed.
On the one hand, numerous techniques have been developed for reducing the PAPR of the OFDM signal.
For example, clipping is used as a PAPR reduction technique, but this can cause in-band and out-of-band distortion. Filtering can then be used to alleviate out-of-band distortion but results in peak re-growth. As such, repeated clipping and filtering can lead to serious degradation in Bit Error Rate (BER), see Armstrong, J., “Peak-to-average power reduction for OFDM by repeated clipping and frequency domain filtering,” Electronics Letters, vol. 38, no. 5 pp. 246-247.
Another approach, windowing, involves multiplying large signal peaks by a non-rectangular window, such as a Gaussian pulse, to minimize the out-of-band interference. Ideally the window should be as narrow band as possible but it should not be too long in the time domain as this means more signal samples are effected, which causes an increase in the BER, see van Nee, R.; de Wild, A., “Reducing the peak-to-average power ratio of OFDM”. 48th IEEE Vehicular Technology Conference, 1998, vol. 3, pp. 2072-2076, 18-21 May 1998.
Windowing has higher BER compared with clipping but produces lower out-of-band distortion.
Selective Mapping (SLM) is another approach. SLM is implemented by generating a set of sufficiently different candidate signals from the original data signal. The transmitter selects and submits the candidate signal having the lowest PAPR.
Partial Transmit Sequencing (PTS) is a similar technique in which sub-blocks of the original signal are optimally combined at the transmitter for generating a transmitter with a low PAPR. Although SLM and PTS are effective at reducing the PAPR, they require the use of side information to the receiver in order to decode the signal at the receiver, see Seung Hee Han; Jae Hong Lee, “An overview of peak-to-average power ratio reduction techniques for multicarrier transmission,” IEEE Wireless Communications, vol. 12, no. 2 pp. 56-65, April 2005.
Pilot tones having the same amplitude are optimal for channel estimation. Hosokawa, S.; Ohno, S.; Teo, K. A. D.; Hinamoto, T., “Pilot tone design for peak-to-average power ratio reduction in OFDM,” ISCAS 2005. IEEE International Symposium on Circuits and Systems, 2005, vol. 6, pp. 6014-6017, 23-26 May 2005 discloses allowing the phase of pilot tones in a OFDM signal to be varied for reducing PAPR. However, the phase values to be used are calculated using an exhaustive search which can involve delay and/or significant processor resources to calculate.
There are also available unused sub-carriers, which normally have a value of zero in order to ensure the spectrum lies within the defined mask. However, Wang et al.—Chin-Liang Wang; Yuan Ouyang; Hsien-Chih Chen, “A peak-to-average power ratio reduction technique for the IEEE 802.11a wireless LAN,” IEEE 58th Vehicular Technology Conference, 2003, vol. 4, pp. 2287-2291, 6-9 Oct. 2003—show that 5 unused sub-carriers in an IEEE 802.11/HiperLAN2 Wireless LAN System can be used for PAPR reduction while keeping the out-of-band distortion well within the spectral mask. This has the effect a slight broadening of the original OFDM symbol and again Wang is based on possibly exhaustive evaluation of combinations of unused carried amplitude and phase values in order to determine an optimal combination for PAPR reduction.
Gatherer, A.; Polley, M. Signals, “Controlling clipping probability in DMT transmission” Conference Record of the Thirty-First Asilomar Conference on Systems & Computers, Vol. 1, Iss., pp. 578-584, November 1997, also propose a PAPR reduction technique employing use of the unused tones. However this is based on continually clipping the signal until a desired threshold is reached. Convergence is very slow and PAPR can often increase following iteration. A comparison between the authors' technique and this is illustrated in FIG. 6.
Several techniques have also been proposed for the removal of near-zero values in the time domain.
Rudolph, D. “Kahn EER technique with single-carrier digital modulations” IEEE Transactions on Microwave Theory and Techniques, Vol. 51, Iss. 2, February 2003 Pages: 548-552 describes a method of cancelling near-zero amplitude values of OFDM signals by adding cancellation signals to the near zero-values in the time domain. However this results in a considerable amount of extra power in the signal and a high BER. Computationally complex ‘hole blowing’ techniques also exist to remove these zero crossings too.
Rudolph and Schaefer, “Method for reducing the out-of-band emission in AM transmitters for digital transmission”, U.S. patent application Ser. No. 10/275,424 also propose a technique which modifies the signal in the frequency domain prior to the operation of the IFFT. In the frequency domain the signal is composed of I and Q components, real and imaginary components. When both the I and Q components simultaneously have zero values a zero or near-zero amplitude results after an IFFT is performed and the signal is converted into the time domain. By adding a pulse to one of the I or Q components in the frequency domain, the near-zero value can be eliminated in the time domain. This helps reduce out-of-band distortion at the output of the polar transmitter.
Robinson and Winter “Modified polar amplifier architecture” U.S. patent Ser. No. 10/719,514, propose an amplifier system that switches between operation in a polar mode. In one aspect of their invention, an amplifier system is provided that includes a power amplifier and a mode selector. The power amplifier is operative to amplify an input signal to provide an amplified output signal. The mode selector controls the operation of the amplifier system in the polar mode based on a characteristic of the input signal relative to a threshold level.
It is an object of this invention to provide a method for PAPR reduction technique as well as removing unwanted near zero values, but which causes no decrease in data throughput, results in very low out-of-band distortion, requires no side information to be transmitted and has a lower BER compared with clipping and windowing.